[Interest] QSyntaxHighlighter change QTextBlockFormat

[Eskil Abrahamsen Blomfeldt]

Hi, Joshua,

I am not sure how option #2 would work in practice. If you edit the document without updating the undo block, then the top command of the undo stack will refer to an outdated version of the document and when you execute it, you would in many cases corrupt the document.

Since all changes have to be recorded in order for the undo stack to make sense, I think the correct approach is to use QTextCursor::joinPreviousEditBlock(). This will append your changes to the top of the undo stack instead of adding a new one. You can group multiple changes together when you are done, use endEditBlock() to mark the end of the edit. Undoing after this will undo your changes together with the previous change.


Eskil Abrahamsen Blomfeldt
Senior Manager, Graphics

The Qt Company
Sandakerveien 116
0484 Oslo, Norway
eskil.abrahamsen-blomfeldt at qt.io
http://qt.io

________________________________
Fra: Interest <interest-bounces at qt-project.org> på vegne av Joshua Grauman <jnfo-d at grauman.com>
Sendt: onsdag 5. januar 2022 05:30
Til: interest at qt-project.org <interest at qt-project.org>
Emne: [Interest] QSyntaxHighlighter change QTextBlockFormat

Hi all,

I have been researching how to have a QSyntaxHighlighter change
QTextBlockFormat. I need my syntax highlighter to change the indent, left
and right margins, line height, etc. of a QTextDocument/QTextEdit.

I was able to have my class derived from QSyntaxHighlighter change the
QTextBlockFormat of individual blocks easy enough using the QTextCursor
interface, but the problem with that is that it messes up undo/redo
functionality. All of the block format changes go onto the undo/redo stack
and so make undo/redo unusable.

I have tried to find a way to modify QTextBlockFormat directly (getting
the QTextLayout * of the current block and trying setPosition() on it, or
even trying to move the individual lines of the QTextLayout), but it can't
modify it here as I suspected (at least the way I tried it).

I can't find any other way to modify QTextBlockFormat in a way that won't
be recorded onto the undo/redo stack.

So my options are (that I can see):

1) Roll my own complete undo/redo stack for QTextDocument/QTextEdit. This
    seems overkill since it's already written.

2) Modify Qt to be able to disable the undo/redo stack temporarily while
    the QSyntaxHighlighter is changing the block format. There's already a
    function QTextDocument::setUndoRedoEnabled(bool enable). But I can't
    use it directly because it clears the undo/redo stack (the docs says
    the undo stack, but the code looks like it's clearing the redo
    stack???). Either way I need to modify
    QTextDocument::setUndoRedoEnabled(bool enable) so that it can enable
    and disable undo/redo without clearing the stack(s). It would seem like
    I could add a second parameter to the function with a default value to
    work like it does currently, but with an option that can be added to
    not clear the stack. Although I've only written 2-3 patches for Qt,
    this seems easy enough that I could do it. But I wanted to check first
    if this seems like a reasonable approach and would be likely to be
    accepted.

Obviously, I would like to take option 2, but I wanted feedback first.
Does that seem like a reasonable approach? Will it be likely to be
included in Qt? I don't want to write a patch if there are clear problems
with the approach. Also, if anyone has a way to easily modify the
blockformat of a block without affecting undo/redo, that would be great.
Or even if someone has rolled their own undo/redo for QTextDocument, I
suppose I could use that. Thanks for any help, it is greatly appreciated.

Josh
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